The advent of the digital computer created a demand for direct-access storage devices capable of storing and retrieving large volumes of data. Main memory (historically referred to as "core memory" but now typically taking the form of "semi-conductor memory") and other fast electronic storage systems were not feasible for mass-storage applications principally because of their costs. Paper, tape and floppy disk memories proved unsatisfactory and ineffective due to their slow access times. Accordingly, digital storage devices using rotating, rigid, magnetic media ("disk drives") were developed as an effective compromise between reasonable information access times, and cost-effective storage capabilities. These disk drives also provided greater storage capacities for a given enclosure volume than did most competing storage devices.
Disk drives typically contain one or more rotating disks which have thin magnetic layers on their planar surfaces. Information normally is stored on and retrieved from the magnetic layers by means of a "flying head", which takes the form of an electromagnetic transducer element and an air-bearing slider. The slider positions the associated transducer on a pressurized air film at a relatively close and constant distance above the rotating disk surface. The pressurized air film is developed by loading a precisely shaped slider against a moving disk surface. A region of the air film moving with the disk is compressed by the slider, thus creating an air pressure that tends to force the slider away from the disk's surface. By carefully controlling the shape and dimensions of the slider and load force acting on the slider, the air being compressed between the slider and the disk creates an upward pressure on the slider which maintains it in equilibrium at a reasonably stable distance away from the disk's surface. Although this technique has traditionally been referred to as "flying head" technology, the term "flying" is a misnomer, inasmuch as the head does not actually fly, but rather is supported by a hydrodynamically lubricated air bearing.
Heads typically are mounted on support arms that are attached to a high-speed actuator. An actuator, essentially, is a support structure for the heads, with the actuator also including a motor (typically referred to, as indicated earlier, as a voice-coil motor) assembly that allows the arm in the actuator to move and to position the heads accurately with respect to certain predetermined positions relative to a disk's surface(s). The most common actuators are either linear or rotary designs. Linear actuators move and position the associated heads on a radial path with respect to the disk's center, whereas rotary actuators, which consist of a pivoted support beam and motor mechanism, move and position the heads in an arced path typically approximating a radial path with respect to the disk center. By positioning the heads selectively at different radii with respect to the axis of rotation of the disk, information can be recorded in discrete, concentric tracks. Since the heads move in unison across the disk's surfaces, all of the heads controlled by a common actuator are maintained in a "cylinder" of tracks. This arrangement permits any track in the "current cylinder" to be accessed within microseconds.
For low-performance disk drives, actuator positioning is performed "open loop", meaning that the actuator's position is determined by a device, such as a stepper motor, with no positional feedback provided from the disk. Open-loop methods limit areal density because they can only be used at relatively low track densities (which are measured in tracks per inch or "TPI"). In contrast, current high-performance disk drives utilize "closed-loop" servo-positioning techniques to read and follow servo-information which is stored on disks. This approach offers significantly greater accuracy in positioning the actuator relative to the information recorded on a disk. Traditionally, in drives with three or more disks, the actuator's position is established with respect to a dedicated disk surface on which servo information is recorded, and all of the heads associated with the actuator stack are positioned in a cylinder relative to the position of the servo head on that dedicated surface. Alternatively, on drives with one or two disks, or on very high-performance drives, servo information is embedded within the data tracks, and head positioning is performed relative to the specific track of information being written or read.
Many computer operating systems today depend upon the availability of reasonably priced, high-performance, mass-storage devices in order to implement practical solutions to such fundamental problems as the limited capacity of relatively expensive main memory. By swapping, or paging, portions of main memory selectively to and from a high-performance disk drive, the drive can be used, in effect, as an extension of main memory. This, in turn, permits a computer to operate on programs and data that greatly exceed the size limitations of actual main memory. Graphical user-interface and multi-media applications create even greater demands for improved disk-drive performance and capacity, and in this kind of a setting, the actuator proposed by the present invention offers important, contributive, enhancement applicability.
In modern computer systems applications, among the most important disk-drive performance parameters are (1) formatted box storage capacity per unit of volume (a measure of volumetric efficiency), (2) data transfer rate, and (3) average actuator access time--each of which parameters is significantly affected by actuator performance.
The average actuator access time is defined in terms of the sum of the average seek time plus the so-called settling time. This access time can be reduced by improving servo-system performance, by improving the performance of the actuator drive motor, by reducing bearing friction, by reducing the moment of inertia of the actuator/drive-motor/head/flexure assembly, etc., and since the average access times of even the most advanced, rigid-disk drive systems are measured in milliseconds, as opposed to the nanoseconds used to measure CPU and semi-conductor memory cycle times, disk-drive accesses are a significant bottleneck in overall computer-system performance.
Similarly, actuator access time operates as an appreciable limit on the achievable, sustained data transfer rate. Although common measures of data transfer rate are typically determined without reference to actuator performance (i.e., peak data rate as determined by the number of bytes in a data track times the angular speed of the disk), non-trivial transfers of data typically require that the head(s) be moved during the course of the data transfer, whenever more than a single track of data is requested. Accordingly, actuator performance has a significant impact on the maximum sustainable data rate.
Volumetric efficiency is also affected by actuator design. In particular, smaller, more compact actuators require less space-"overhead" within a disk-drive housing. Space constraints have become particularly problematic in portable, rigid-disk drive applications, where disk diameters are now only about 2-inches, and where the permissible overall drive height is only about 1/4-inch. Because rotary actuators are generally more compact than linear actuators, are usually less complicated in design, and are more easily mass balanced, rotary actuators tend to predominate in this small drive regime. Nonetheless, it remains a significant challenge to design a high-performance rotary actuator within the confines, say, of a 48-millimeter-form-factor disk drive.
In addition to the space limitations just mentioned above, advanced actuators for use in small-form-factor drives are also required to maintain relatively high levels of performance, notwithstanding reductions in available power, and also not-withstanding reduced voice-coil motor dimensions--thus placing a significant premium on actuator efficiency.
The trend toward small disk-drive designs has also been accompanied by a corresponding trend toward smaller, extremely low-mass heads which are designed to operate very near to, or in extremely-low-load-contact with, the associated disk surface. Such reduced head mass necessitates a corresponding reduction in the mass of the voice-coil assembly (situated on the opposite end of the actuator) in order to maintain an appropriate dynamic balance about the actuator's axis of rotation, which balance is required to minimize the effects of radial forces that will otherwise tend to urge the head(s) off track. For extremely low-mass head/suspension systems used in conjunction with prior art actuators in drives having few heads per actuator, however, the mass of the voice-coil side of the actuator structure (relative to the actuator's axis of rotation) is likely to exceed the mass of the transducer-carrying portion of the actuator. Therefore, in order to maintain dynamic mass balance in such an actuator assembly, additional mass must typically be added to the transducer side of the actuator. Unfortunately, such additional mass increases the moment of inertia of the actuator, and consequently decreases its resonant frequency--resulting in adverse effects on seek time and servo-system performance.
Yet another consideration with regard to the issue of low-mass actuator assemblies is that there are components on these assemblies, for example, a drive-motor winding, and the energizing/pick-up coils associated with transducers, to which conductive connections must be made with external circuitry. Accordingly, conductors must extend between such components and the "outside world", and these conductors must accommodate the relative motion which occurs between such an assembly and the associated main frame structure during normal operation. A consequence, of course, is that, typically (considering prior art approaches), the extending, connective conductors offer resistance to motion which can cause a number of very undesirable effects, such as, for example, biasing an actuator undesirably toward one of its limits of travel, and adding a force that must be overcome during actuator operation which can slow down actuator positioning performance, and/or require a larger drive motor and/or the application of more drive-motor power. For example, a typical, traditional approach is to use flex-circuit conductors which extend in a reverse bend toward connections on an actuator's moving structure, the manipulation of which bend requires continuous, "offsetting" servo power.
The various problems presented by such "outside world" conductive connections have been addressed in different ways in prior-art approaches toward providing solutions, and, as an example, this very issue is discussed in U.S. Pat. No. 4,476,404 to Bygdnes, U.S. Pat. No. 5,025,335 to Stephansky, and U.S. Pat. No. 5,025,336 to Moorehouse et al. These three patents disclose different approaches toward a currently favored way of reducing the problems just mentioned-namely, the use of so-called ribbon-like flex circuits that extend between an actuator assembly and the obligatory outside-world circuitry. Nonetheless, these kinds of solutions, as illustrated in these three patents, require some significant moving of connecting conductors, and attendant overcoming of resistance forces, and do indeed leave room for thoughtful improvement.
Accordingly, the present invention described, illustrated and claimed herein provides a compact, high-performance rotary actuator (and related structure in an assembly including a read/write transducer) which significantly overcomes the problems inherent in the prior art, and which specifically addresses the important actuator-system structural and performance considerations discussed above. In addition, it involves a structural organizational method which assures successful addressing of such problems and considerations.
In the context of illustrating and describing the present invention, one should recognize that significant recent advances over prior art disk-drive technology have occurred, and several important ones of these, which are related in different ways to the features and advantages of the present invention, have been disclosed in the following U.S. patents and co-pending U.S. patent applications:
1. U.S. Pat. No. 5,041,932 for INTEGRATED MAGNETIC READ/WRITE HEAD/FLEXURE/CONDUCTOR STRUCTURE, issued Aug. 20, 1991; PA0 2. U.S. Pat. No. 5,073,242 for METHOD OF MAKING INTEGRATED MAGNETIC READ/WRITE HEAD/FLEXURE/CONDUCTOR STRUCTURE, issued Dec. 17, 1991; PA0 3. U.S. patent application Ser. No. 07/710,561 for INTEGRATED MAGNETIC READ/WRITE HEAD/FLEXURE/CONDUCTOR STRUCTURE, filed Jun. 5, 1991, now U.S. Pat. No. 5,111,351, issued May 5, 1992; PA0 4. U.S. patent application Ser. No. 07/710,891 for INTEGRATED MAGNETIC READ/WRITE HEAD/FLEXURE/CONDUCTOR STRUCTURE, filed Jun. 11, 1991, now U.S. Pat. No. 5,163,218, issued Nov. 17, 1992; PA0 5. U.S. patent application Ser. No. 07/684,025 for WEAR-RESISTANT HEAD FOR CONTACT READING AND WRITING MAGNETIC MEDIA, filed Apr. 10, 1991; PA0 6. U.S. patent application Ser. No. 07/746,916 for UNITARY MICRO-FLEXURE STRUCTURE AND METHOD 0F MAKING SAME, filed Aug. 19, 1991; PA0 7. U.S. patent application Ser. No. 07/760,586 for HIGH-CAPACITY, MICRO-SIZE, RIGID-DISK, MAGNETIC DIGITAL-INFORMATION STORAGE SYSTEM, filed Sep. 16, 1991; PA0 8. U.S. patent application Ser. No. 07/783,509 for SIZE-INDEPENDENT, RIGID-DISK, MAGNETIC, DIGITAL-INFORMATION STORAGE SYSTEM WITH LOCALIZED READ/WRITE ENHANCEMENTS filed Oct. 28, 1991; and PA0 9. U.S. patent application Ser. No. 07/783,619 for GIMBALED MICRO-HEAD/FLEXURE/CONDUCTOR ASSEMBLY AND SYSTEM.
The respective disclosures of these documents are now expressly incorporated by reference into the present disclosure.